The importance of interleukin-8 as a neutrophil chemoattractant in the lungs of cattle with pneumonic pasteurellosis

Interleukin-8 (IL-8), an in vitro and in vivo neutrophil chemoattractant, i s expressed at high levels in the lesions observed in bovine pneumonic past eurellosis. Because of the role of neutrophils in the pathogenesis of pneum onic pasteurellosis, we investigated the relative importance of IL-8 as a n eutrophil chemoattractant in this disease. Bronchoalveolar lavage (BAL) flu id was harvested from calves experimentally infected with bovine herpesviru s-1 and challenged with Mannheimia haemolytica. Neutrophil chemotactic acti vity was measured in pneumonic BAL fluid samples treated with a neutralizin g monoclonal antibody to ovine IL-8, and compared to the activity in sample s treated with an isotype-matched control antibody. Bronchoalveolar lavage fluid was analyzed at a dilution which induced a half-maximal response, and the concentrations of antibody were optimized in a preliminary experiment. Following incubation of replicate samples of diluted pneumonic bovine BAL fluid with 70 mug/mL of IL-8-neutralizing antibody or control antibody, the neutrophil chemotactic activities of the samples were determined using an in vitro microchemotaxis assay. Overall, pretreatment of BAL fluid samples with neutralizing anti-IL-8 antibody reduced neutrophil chemotactic activit y by 15% to 60%, compared to pretreatment with control antibody. This effec t was highly significant (P < 0.001), and was present in 5 of 5 samples. Th ese data indicate that IL-8 is an important neutrophil chemoattractant in c alves with pneumonic pasteurellosis, but that mediators with actions redund ant to those of IL-8 must also be present in the lesions.